In case of a nitride semiconductor epitaxial layer that is grown on a substrate, lots of crystal defects occur due to a mismatch of the lattice constant and a difference in the coefficient of thermal expansion. Warpage of the substrate is caused by internal stress. Moreover, the warpage of the substrate acts as hindrance in fabricating a large area substrate.
In order to overcome such mismatch between the substrate and the nitride semiconductor epitaxial layer, U.S. Pat. No. 4,855,249 discloses a technology in which an AlN buffer layer is grown on a sapphire substrate at low temperature and an AlxGa1-xN (0≦x<1) layer is then grown. Further, U.S. Pat. No. 5,290,393 discloses a technology in which an Al1-xGaxN (0<x≦1) buffer layer is grown on a sapphire substrate at a temperature ranging between 200° C. and 900° C. and an Al1-xGaxN (0<x≦1) layer is then grown at a temperature ranging between 900° C. and 1150° C.
Furthermore, U.S. Pat. No. 6,051,847 discloses a technology in which in a process wherein a buffer layer containing indium is grown on a sapphire substrate and the GaN based compound semiconductor layer is then grown on the buffer layer, indium contained in the buffer layer is diffused into a GaN based compound semiconductor layer.
Meanwhile, in order to solve such mismatch, technologies using a substrate made of GaN have been proposed. However, these methods have a problem in that light emitting devices using the GaN substrate have not yet been commercialized due to difficulty in growing a GaN bulk and a high cost spent in the growth.